This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Background &Aims: Regulation of gene expression in the Lyme disease spirochetes is of fundamental importance to establishing a successful infection. In this study, we investigated the functional elements of the flaB promoter of Borrelia burgdorferi. Our goal was to understand the basis for promoter discrimination in this organism. Promoters constitute one of the basic determinants of differential gene expression in this organism. Methods: Promoter function was examined in a high-passage variant of strain JD1 using a set of 5'deletions and mutations within the flaB promoter. Expression from the modified flaB promoters was assayed using the gene for green fluorescent protein (gfp) as a reporter. Results: Although the -35 element of the promoter stimulated promoter activity, its disruption did not negate expression. Sequence upstream of the -35 had no effect on expression. The -35/-10 spacer region composed of a T-rich sequence was critical for optimal promoter function. Surprisingly, a Cytosine at the -13 site was found to be more favorable for transcription compared to a Guanosine at the same site. Conclusions: Based on these results and other characteristics, we propose that the B. burgdorferi flaB promoter is an example of an extended -10 promoter. Further, the T-rich spacer is a key element of the flaB promoter that contributes to the abundance of the flagellar core protein in Borrelia species.